Patent document 1: JP-A-2006-44430
Patent document 2: JP-A-2006-27368
Patent document 3: JP-A-2006-44449
Patent document 4: JP-A-2001-322554
Patent document 5: JP-A-2001-146169
Patent document 6: JP-A-2002-96749
In an electric power steering system for an automobile for assisting the steering operation, a speed reducer which consists of a worm shaft serving as a driving gear and a worm wheel serving as a driven gear meshing with the worm shaft is used for a transmission which transmits the power of an electric motor for steering assistance to a steering mechanism.